Corrosion inhibition of trichloroacetates



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v Patented May 29, 1951 CORROSION INHIBITION OF TRICHLORO- ACETATES Francis N. Alquist and Joseph L. Wasco, Midland,

Mich., assignors to The Dow Chemical Company, Midland, Mich., a corporation of Dela- Ware No Drawing. Application July 15, 1950, Serial No. 174,128

12 Claims. 1

This invention is concerned with trichloroacetates and is particularly directed to an inhibited composition of a water-soluble salt. of trichloroacetic acid, and to a method for preventing the corrosion of metals in contact with the trichloroacetates.

The water-soluble salts of trichloroacetic acid have been employed as vegetation destroyers and particularly as weed killers for several years. While remarkably effective for the control of many species of noxious vegetation, these compounds and their aqueous solutions are somewhat objcctionable by reason of being corrosive to metals. Thus, problems are encountered with tank cars, drums and spray equipment, and with metal structures around which the compounds are commonly applied as herbicides. For example, tank cars and drums in which aqueous solutions of trichloroacetates are transported may show pitting and disintegration of interior surfaces. Sp-ray equipment and tanks in which solutions of the products are employed, particularly when made of light metal, may be corroded and caused to develop leaks. Fences, metal rails and posts, tower installations and the like may be caused to pit and rust unless precautions are taken to avoid contact with the trichloroacetates as applied. In spite of these difficulties, the latter chemicals are being widely employed in weed control programs, and their great utility makes essential the provision of means for controlling the corrosion problem.

It is among the objects of the present invention to provide means for minimizing the corrosion of metal surfaces by compositionscomprising salts of trichloroacetic acid. A further object is to provide compositions which contain addita ments serving as corrosion inhibitors. An additional object is to provide both dust compositions and aqueous solutions of water-soluble salts of trichloroacetic acid containing corrosion inhibitors. Still another object is to provide a method whereby the corrosive action of the trichloroacetates is largely controlled. Other objects will become evident from the following specification and claims.

In accordance with the present invention the corrosion of metals by water-soluble salts of trichloroacetic acid is inhibited by dispersing in the trichloroacetate an alkali metal salt of a chromium acid. An improved trichloroacetate product is thereby obtained which, although contacted with metals in either the powdered form or in the presence of water (as in an aqueous solution), causes but a small percentage of the corrosion 2 ordinarily resulting from a similar contact wit the trichloroacetate alone. The expression alkali metal salt of a chromium acid as herein employed refers to sodium, potassium, and ammonium chromates and dichromates and the hydrates of these compounds. In preferred embodiments of the invention, such chromium compound may be supplemented with either watersoluble inorganic alkali or a water-immiscible high-boiling oil or both, the exact choice of additament or combination of additaments being largely determined by the particular corrosion problem under consideration.

The problems of corrosion take many forms. Thus, in tank cars, tanks and drums in which aqueous solutions of the trichloroacetates may be transported or stored, the metal surfaces may be attacked either below, above or at the liquid level of the solution. In spraying equipment, not only the tank but also the fittings, pumps and other metal appurtenances are vulnerable to corrosion. With dry compositions, drums and the metal parts of mechanical dusters may undergo considerable damage upon contact with the composition. Similarly, fences, railroad rails, and metal posts are susceptible to attack. Metals in common use which have been found to require protection include aluminum, tinplate, copper, brass, galvanized iron and mild steel.

The amount of the chromium compound required to accomplish the desired result varies somewhat with the particular salt to be inhibited and the metals to be protected against corrosion. In general, however, about 0.3 percent of the chromium compound based on the weight of the trichloroacetate greatly reduces the corrosiveness of the dry composition and of aqueous solutions prepared from the basic mixture. A proportion of 0.5 percent or more of the chromium compound has been found to have a desirable inhibiting action under most conditions.

In the preferred operation with alkali, the latter is ordinarily employed in amount suificient to adjust the reaction of an aqueous solution of the trichloroacetate to a pH of Hi-10.5. This frequently is about 0.5 to 2.0 percent by weight of the dry weight of the trichloroacetate. When oil is incorporated into the dry trichloroacetate salt, about 1.0 percent by weight is employed. Otherwise, a thin layer is floated on the surface of the aqueous solution.

In preparing the composition of the invention, the chromium compound may simply be ground, or otherwise mechanically mixed, with the trichloroacetate compound. Similarly, the inorganic alkali and oil may be dispersed in and through the trichloroacetate salt in any convenient manner. Alternatively, the chromium compound and alkali may be dissolved in an existent aqueous solution of the trichlorcacetate salt, and the oil floated on its surface, if desired.

The chromium compounds found operable in accordance with the present invention include sodium chromate, sodium dichromate, ammonium chromate, potassium chromate, and hydrates of the foregoing. All of these materials do not operate with equal efliciency to control corrosion of all metals. However, sodium dichromate appeared to be effective in most instances, and the corrosion of aluminum was inhibited by all of the materials enumerated.

Among the water-soluble inorganic alkalies which may be employed in accordance with the present invention are sodium hydroxide, sodium bicarbonate, sodium carbonate, sodium phosphate (mono-, di-, and tribasic), and the corresponding potassium and calcium compounds which do not cause an excessive precipitate in solutions subsequently prepared from the trichloroacetate salt mixtures. Suitable waterimmiscible oils for inclusion in the compositions as described are high boiling mineral oils, corn oil, cottonseed oil, or other oily materials which are stable in the presence of the other constituents of the mixtures set forth.

The following examples illustrate the invention but are not to be construed as limiting:

Example 1 Test strips were prepared from sheet stock of mild steel and of aluminum (38). The steel strips were 2.75 x 1.0 x 0.125 inches in measurement, having a surface area of 6.43 square inches. The aluminum strips measured 2.75 x 0.5 x 0.0625 inches and had a surface area of 3.156 square inches. These test strips were cleaned, carefully weighed, and suspended one-half immersed in closed glass containers partially filled with an aqueous solution of percent by weight of the isopropylamine salt of trichloroacetic acid and 0.6 percent by weight of sodium dichromate as an inhibitor. Check determinations were made with an aqueous 10 percent solution of the trichloroacetate in the absence of the dichromate. After seven days exposure at room temperature, the metal strips were removed, Washed with water, carefully cleaned of adhering corrosion products, dried, and reweighed to determine weight loss. The following are representative of the results obtained.

Weight In tial of Strip, Weight Meta] We1gl 1t inGrams, Loss,

of Strip, Followin in Grams ing Ex- Grams posure Al (uninhibited) 3. 428 3. 381 0. 047 2. 838 2. 780 0. 058 3. 575 3. 574 0. 001 3. 139 3. 137 0. 002 46. 511 45. 780 0.731 46. 164 45. 403 0. 761 46. 066 46. 642 0. 024 46. 236 46. 205 0. 031

Example 2 Sodium trichloroacetate compositions inhibited with 1.2 percent by weight of several different chromium compounds were separately dissolved in water to obtain a series of aqueous solutions containing 50 percent by weight of the trichloroacetate and 0.6 percent of inhibitor. These solutions were compared with an unmodified 5 0 percent by weight aqueous solution of sodium trichloroacetate to determine corrosion against aluminum. The mode of operation was substantially that described in Example 1, except that the exposure time was eight days. The aluminum strips were as described in the preceding example. The following results were obtained:

In a similar fashion test strips of mild steel as described in Example 1 were exposed over a period of 21 days to the action of an aqueous solution of inhibited sodium trichloroacetate. This solution was in the proportion of parts by weight of sodium trichloroacetate, 0.4 part by weight of sodium dichromate, and 100 parts by Weight of water. Check determinations were carried out with an uninhibited 50 percent trichloroacetate solution. The following results were obtained.

Weight of lntial Srip, in Ileight 4 trip rams oss 501mm Weight, Follow in in Grams ing Ex- Grams posure Uninhibited 46. 578 46. 397 0. 181 D 44. 928 44. 772 0.156 46.135 46. 064 0.071 46.335 46. 267 0. 069

Other determinations were carried out with test strips of aluminum (38) which were 2.75 x 1.0 x 0.125 inches in dimension and having a surface area of 6.43 square inches. The same basic 50 percent sodium trichloroacetate solution was employed as with the steel strips, but varying amounts of sodium dichromate were employed. The following results were obtained after 29 days exposure at room temperature:

Strips were cut from aluminum sheeting (SAE24) to the size of 2.75 x 0.5 x 0.0625 inches and having a surface area of approximately 3.156 square inches. These pieces were cleaned,

Weight of Initial Strip, in Loss-of Weight Grams, Weight, of Strip, Followin in Grams ing Ex- Grams posure N inhibitor a 3. 868. 3 .761 0.107 D 3. 845 i 614 0. 231 3.84 5 3 844 0.001 3. 904 3 901 0.003

Example 5 Test strips were cut from sheets of 85-15 red brass (2.5 x 1.0 x 0.0625 inches), 70-30 yellow brass (2.5 x 1.0 x 0.0312 inches), 99.9 percent copper (2.5 x 1.0 x 0.0625 inches), and tinned iron (3.5 x 0.5 x 0.0625 inches) and exposed to the corrosive action of a 50 percent by weight aqueous sodium trichloroacetate solution substantially as described in Example 1. In this operation, the basic solution was employed in the checks, and compared to an inhibited composition containing on the dry weight basis of sodium trichloroacetate 0.7 percent each of sodium dichromate and trisodium phosphate. After an exposure of 21 days, the test strips were washed, cleaned, dried and reweighed to obtain the following results:

Similar determinations were carried out with galvanized nails. These were cleaned as well as their irregular surfaces would permit and thereafter suspended in both the uninhibited and the inhibited solutions as described above. After v21 days, the nails were examined. All were found'to have been attacked to some degree. However, those suspended in the inhibited solution appeared to be significantly less corroded than were those exposed to the action of the unmodified sodium trichloroacetate solution.

Example 6 Mild steel sheeting was cut into strips as described in Example 1 and subjected to the action of a '50 percent by weight aqueous solution of sodium trichloroacetate and of an inhibited portion of the latter. The method of determination was substantially that reported in Example 1. The inhibited composition contained 0.45 percent of sodium dichromate and from 0.16 to 0.8 percent of sodium hydroxide based on the dry weight of sodium trichloroacetate in the solution. In addition, each stabilized solution was covered by a one-eighth inch surface film of refined white mineral oil approximating medicinal grade. Twenty-eight days following the exposure of the steel strips, the latter were removed, cleaned. dried and weighed. The following data was obtained:

Weight Initial of Strip, Weight Per Cent by Weight of NaOH igfigg f' gi in Grams ing Ex- Grams posure Check (no chromate, NaOH, or oil) 8: 0 16 45.676 46. 654 0.022 46. 600 5 '46. 545 O. 055 46. 517 46. 491 0.026

Example 7 A formulation was prepared in which sodium trichloroacetate was ground and mixed with sodium carbonate and sodium dichromate. This production was on a tonnage basis with the proportion of sodium carbonate varying between 0.5 and 2.0 percent by weight, and the sodium dichromate being employed at 0.5 percent by weight, both based on the weight of sodium trichloroacetate in the mixture.

The compositions as obtained were widely distributed for use in the killing of unwanted vegetation. In such application, a common procedure included dissolving the inhibited sodium trichloroacetate in water to produce relatively dilute aqueous solutions. The latter have been employed in many types of spray rigs and applicators, and so brought into contact with tanks, pumps, nozzles and other working parts constructed of a wide variety of metals. It has been observed that these spray solutions are markedly inhibited as regards the corrosion of metal surfaces. In contrast, uninhibited solutions of sodium trichloroacetate have been found to be very corrosive, and their use in conventional spray rigs to cause much damage to such apparatus.

Example 8 A standard commercial composition consisting of a 50 percent by weight aqueous solution of sodium trichloroacetate was shipped in mild steel tank cars and stored in the cars for periods of 7 from 30 to 60 days. At the end of such period, the interiors of the tank cars were so pitted and otherwise corroded as to preclude the continuation of the practice.

A quantity of the foregoing material, having a reaction of pH 4-, was modified with 0.5 percent by weight of sodium dichromate and sufficient sodium carbonate to impart a reaction of pH 8 to the mixture. This was accomplished by first adding the sodium dichromate and thereafter the sodium carbonate, both with adequate mixing and stirring. The resulting mixture was found by laboratory determination to be relatively noncorrosive to mild steel and aluminum.

This inhibited mixture was then shipped in standard SOQO-gallon tank cars of mild steel construction. In loading the cars, 55 gallons of a highly aromatic petroleum oil (marketed as AMSCOHT) was first introduced into each car, and the sodium trichloroacetate composition thereafter charged into the cars in the conventional manner. Photographs of the interior of a representative car were taken prior to loading. This car was returned to the loading dock 39 days after loading. The interior of the car was washed with water, dried and again photographed. There was no evidence of pitting or other corrosion of the interior surfaces attributable to contact with the inhibited acid solution.

We claim:

1. A composition of matter comprising (1) a water-soluble salt of trichloroacetic acid and (2) as a corrosion inhibitor therefor an alkali metal salt of a chromium acid.

2. A composition according to claim 1 in which the chromium compound is present in the amount of at least 0.3 percent by Weight of the trichloroacetate.

3. A composition according to claim 1 in which the water-soluble trichloroacetate is sodium trichloroacetate.

4. A composition according to claim 1 in which the water-soluble trichloroacetate is calcium trichloroacetate.

5. A composition according to claim 1 in which the water-soluble trichloroacetate is isopropyl--v amine salt of trichloroacetic acid.

6. A composition according to claim 1 in which the corrosion inhibitor is sodium dichromate.

7. A composition according to claim 1 in which the corrosion inhibitor is sodium chromate.

8. A composition of matter comprising a watersoluble salt of trichloroacetic acid, and as a corrosion inhibitor therefor (1) an alkali metal salt of a chromium acid and (2) a water-soluble inorganic alkali.

9. A composition of matter comprising a we.- ter-soluble salt of trichloroacetic acid, and as a corrosion inhibitor therefor (1) an alkali metal salt of a chromium acid, (2) a water-soluble inorganic alkali, and (3) a water-immiscible oil;

10. A powdered composition comprising as a major constituent a solid water-soluble salt of trichloroacetic acid, and at least 0.5 percent by weight each of sodium dichromate and an inorganic alkali based on the dry weight of the trichloroacetate, such composition being inhibited against corrosion of metals both in the dry state and in aqueous dispersion.

11. An aqueous solution of a Water-soluble salt of trichloroacetic acid comprising a minor proportion of at least 0.5 percent by weight of sodium dichromate based on the weight of the solution, sufficient sodium carbonate to adjust the reaction of the solution to pI-I.7.0-l0.5, and a layer of high boiling petroleum distillate floating on the surface thereof.

12. A method for minimizing the corrosive action against metals of water-soluble salts of trichloroacetic acid which comprises the step of dispersing in the trichloroacetate composition at least 0.3 percent by weight of alkali metal salt of a chromium acid.

FRANCIS N. ALQUIST. JOSEPH L. WASCO.

No references cited. 

1. A COMPOSITION OF MATTER COMPRISING (1) A WATER-SOLUBLE SALT OF TRICHLOROACETIC ACID AND (2) AS A CORROSION INHIBITOR THEREFOR AN ALKALI METAL SALT OF A CHROMIUM ACID. 